1. Field of the Invention
This invention relates to fastener assemblies and, more particularly, to such assemblies which are used to secure insulation and membrane to a building structure.
2. Description of the Prior Art
When roofing is installed on commercial buildings, a layer of insulation is normally secured to the roof deck. The assemblies which are used to secure the insulation generally comprise a washer-like stress plate made of either plastic or metal which receives a screw-like fastener that is threaded into the roof deck, clamping the insulation between the stress plate and the roof deck. Since insulation utilized is commonly up to six inches thick, while special applications can exceed twelve inches, it is important to keep the fastener in perpendicular alignment when installing the plate so that the stress plate properly contacts the roof deck. To facilitate this alignment, the plate and fastener are commonly installed using an installation tool, such as that described in U.S. Pat. No. 4,809,568.
An additional problem with the stress plate, screw, insulation, and roof deck assembly is vibration causing the fastener to back out of the stress plate. This will allow loss of the clamp load on the insulation which could result in destruction of the roofing system. Another problem with screw back-out is the possibility of the screw head penetrating the waterproofing membrane of the roofing system allowing water to enter the roofing system. This problem has been addressed by locking and alignment devices in plastic stress plates: see U.S. Pat. No. 4,361,997.
Plates utilized in the past to secure insulation have generally been made solely of either metal or plastic. While plastic stress plates have the advantage of using the above-noted locking and alignment devices, they may not have the advantages of steel stress plates in several applications. In roofing membrane applications where propane fueled torches are used, there may be occasions where, if done improperly, a plastic stress plate which is commonly made of polypropylene or polyethylene could reach a temperature where the clamp load on the insulation between the stress plate and metal deck could partially release. Polymer materials are available that would prevent this; however, their cost is prohibitive to use on the total stress plate. An ideal composite stress plate would enable an inexpensive coated steel to be used for the majority of the stress plate with only the central hub produced from the high performance polymer.
In some applications, all-plastic plates have been found inadequate for providing peripheral support to the insulation around the point of attachment of the plate due to the fact that the outwardly extending portion of the plate is not as rigid as that of its all-metal counterpart. Thus, all-plastic plates have frequently been designed with corrugated ribs, etc. to strengthen rigidity. See, for instance, U.S. Pat. Nos. 4,520,606 and 4,726,164. The necessity of adding sufficiently sized ribs to the plastic plate structure could increase fabrication costs and thus threaten to reduce demand in the construction industry for all-plastic plates. With a composite stress plate, an inexpensive coated metal stamping could be used for the required peripheral support while allowing a plastic insert for the advantages of alignment and head retention.
As mentioned above, alignment of the fastener and the plate during installation is important as insulation thickness frequently requires use of screws up to seven inches long, and over twelve inches long in special applications. Thus, it is desirable to stabilize the plate on the installation tool in proper alignment with the fastener. It is also desirable to prevent relative rotation of the plate on the tool during installation as this frequently causes misalignment of the fastener.
Recently, others in the art have utilized composite two-plate structure to secure insulation to roof deck. U.S. Pat. No. 4,757,661 discloses a washer with axial ribs for attaching insulation to a roof member. The fourth embodiment of this invention is comprised of two components, a central body and a flange, and they are shown as being made of two different materials, plastic and metal. Antirotational means are also disclosed, but the problem of fastener and plate alignment is not addressed.
Therefore, it is an object of the present invention to provide a plate for securing insulation to a roof deck which incorporates the advantages of plastic and metal construction in one composite structure, minimizing the disadvantages associated with such plates which are all-metal or all-plastic. Additionally, it is an object of the present invention to improve alignment of the fastener when installing the washer-like plate and to insure that the clamp load of the fastener on the plate is maintained throughout the life of the roof structure. Finally, it is an object of the invention to prevent rotation of the plate on the installation tool when the threaded fastener is being installed, and to prevent relative rotation between the plastic insert and the metal stress plate.